1. Field of the Invention
The present invention relates to a system and method for managing a powertrain in a vehicle.
2. Background Art
Although four-wheel-drive vehicles may provide much better traction than their two-wheel-drive counterparts, four-wheel-drive vehicles typically sacrifice fuel economy. To help mitigate the increase in fuel consumption in four-wheel-drive vehicles, driveline disconnect devices may be used to selectively disconnect one of the axles, temporarily allowing the vehicle to effectively operate in a two-wheel-drive mode. Thus, the use of driveline disconnects helps a vehicle to regain some of the overall fuel economy that may otherwise be lost because of its operation in a four-wheel-drive mode.
Another way that fuel economy in vehicles is being increased is through the use of hybrid electric vehicles (HEV's). These vehicles typically have a moderately sized internal combustion engine and one or more electric motors, each of which may be capable of providing torque to drive the vehicle. One of the ways in which HEV's increase fuel economy is through the use of regenerative brake energy recovery. That is, one or more of the electric motors may provide resistance to slow the vehicle. The motor or motors are effectively driven as generators, outputting energy to a storage device, such as a battery. The energy stored in the battery is later used to drive the motors during certain vehicle operating conditions.
In the case of an HEV that is a also a four-wheel-drive vehicle, the use of driveline disconnects is at odds with the regenerative braking system. This is because an axle needs to be connected to the powertrain in order for that axle to participate in the recovery of braking energy. If the driveline is connected to the powertrain, however, fuel economy is reduced because all four wheels are being used to drive the vehicle.
Therefore, there is a need for a system and method for managing a powertrain in a vehicle such that under certain vehicle conditions only one set of wheels is connected to the vehicle powertrain. This helps to increase the overall fuel economy of the vehicle. Under certain other vehicle conditions, in particular, when regenerative braking is desired, or when the vehicle is in a four-wheel-drive mode, both sets of wheels are connected to the powertrain, thereby maximizing the amount of regenerative braking energy that can be collected.